1. Field of the Invention
The invention is related to a rolling elements, the rolling surface of which is in contact with a raceway of both rings, at least one of the surfaces of the rings and the rolling elements being provided with a topography comprising recesses which are generally isolated from each other by lands and which may contain a lubricant, said recesses having a maximum diameter of 100 micrometer in a direction parallel to the surface in question.
2. Description of Related Art
Such rolling element bearing is disclosed in WO-A-9719279. The object of the invention is to provide a rolling element bearing wherein a good compromise is achieved between a sufficient lubrication capacity on the one hand, and only a small disturbance of the running properties on the other hand.
Said object is achieved in that the recesses have a minimum diameter of 14 micrometer, and in that, in a cross section, the average angle a defined by the intersection of the wall of a recess and the adjacent surface is less than 5 degrees.
The recesses with such size appear to behave as pockets which are flattened during overrolling, resulting in an extra amount of lubricant being expelled and fed into the contact as an EHL (elasto-hydrodynamic lubrication) lubricant film. Thereby, the rolling surfaces will be separated already at a lower speed.
Preferably, the average size of the recesses in a direction parallel to the surface is 50 micrometer. Further, the recesses may have a maximum depth of 4 micrometer; preferably, the recesses have an average depth of less than 0,5 micrometer. The depth preferred is about 0,2 xcexcm.
It appears that the geometry of the recesses is of importance for obtaining a good lubricant film at low speeds. In particular, it appears to be advantageous in case, in a cross section, the average angle defined by the intersection of the wall of a recess and the adjacent surface is less than 2 degrees.
Thereby, an improved lubrication may be obtained in combination with improved fatigue life properties. The average surface area may be more than 150 xcexcm.
In this respect, it is preferred that the recesses are spherically shaped. A spherical geometry provides a well defined recess, having the required lubricant holding capacity and providing the possibility to locate the recesses close to each other in a non-overlapping way, thus maintaining lands which isolate the recesses from each other. Such recesses may be obtained by a peening operation.
According to a first possibility, the topography comprising recesses may be applied to at least one of the rolling surfaces of the rolling elements, and/or at least one of the raceways of the rings.
The topography in a direction transverse with respect to the direction of rolling may be varying. For instance, the depth and/or the density of the recesses (number of recesses per unity of surface area) may be varying. This embodiment of the rolling bearing according to the invention is in particular useful in the case of varying lubrication requirements.
According to a second possibility, the topography comprising recesses may be applied in a roller bearing wherein at least glass beads are used in the shot peening operation.
The invention is furthermore related to a method for manufacturing a rolling element or a ring for a bearing as described before, wherein at least one of the rolling surfaces and the raceways is provided with a topography comprising recesses which are generally isolated from each other by lands and which may contain a lubricant, which topography in a direction transverse with respect to the direction of rolling is varying. Preferably, glass beads are used in the shot peening operation. The diameter of the glass beads should be about 200 micrometer.
The invention will now be described further with reference to the figures.